c++: Add testcase for CWG2577 [PR120778]

c++: Add testcase for CWG2577 [PR120778]

And here is the last part of the paper.  Contrary what the paper claims
(clearly they haven't tried -pedantic nor -pedantic-errors), I think we
already diagnose everything we should.

2025-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR preprocessor/120778
	* g++.dg/DRs/dr2577-1.C: New test.
	* g++.dg/DRs/dr2577-2.C: New test.
	* g++.dg/DRs/dr2577-2.h: New file.
	* g++.dg/DRs/dr2577-3.C: New test.
	* g++.dg/DRs/dr2577-3.h: New file.